Process for obtaining krypton and xenon



Patented Nov. 17, 1936 Heinrich Kahle, Hoellriegelskreuth, near Munich, Germany, assignor, by mesne assignments, to Union Carbide and Carbon Corporation, a corporaflon of New York I No Drawing. Application May 9, 1934, Serial No.

- 724,821. In Germany May 13, 1933 9 Claim.

This invention relates to the separation of rare gases, such as krypton and xenon, from air, having special reference to a novel process for effecting such separation.

Several processes are known according to which the rare gases, especially krypton and xenon, are

' obtained from the oxygen of air separation plants,

or directly from air, by rectification. Certain hydrocarbons, such as methane, ethane, ethylene, and acetylene, which occur in air in minute quantities, become concentrated in solution in liquid oxygen together with krypton and xenon and are removed when their concentration becomes dangerous. The remaining mixture of oxygen, krypton, and xenon is separated into its constituents by further rectification.

Since krypton and xenon are present in the air in the exceedingly small quantities of about 1 part in 1,000,000, and about 1-part in 10,000,000 respectively, very large volumes of air must be processed to obtain appreciable amounts of these rare gases. Since any increase in the pressure of rectification represents a substantial increase in power consumption, it is desirable and has heretofore been the practice to carry out the rectification of air or oxygen containing krypton and xenon at the lowest possible pressure, usually approximately atmospheric. This is especially the case because ordinarily the boiling curves of the constituents of mixtures are farther separated at lower pressures and rectification is thus more easily effected.

However, it was noticed in these prior processes that certain diificulties arose as the concentration of the solution of krypton and xenon in oxygen was increased, and it has now been found that krypton and xenon are not infinitely miscible with oxygen in liquid form, but that, at higher concentrations, the rare gases are separated in solid form at the pressures heretofore used for rectification. This separation in solid form occurs at'a mixture ratio which lies between pure krypton and xenon and the concentration of the starting mixture, so that the solid krypton and xenon are precipitated in the rectifying column, obstructing the column and interfering with or halting the rectification.

Therefore, it is the principal object of the present invention to provide a process for obtaming k pton and xenon which avoids these difllculties.

I have found-that the gap in the miscibility of liquid krypton and xenon with oxygen before referred to is narrowed under increased pressures and disappears entirely above a pressure of about 1.7 atmos. gage, so that liquid krypton and xenon become infinitely miscible with liquid oxygen at such pressures. Hence, in accordance with this invention, the rectification of the mixture of oxygen and krypton and xenon is performed 5 under increased pressures, and preferably under a pressure above about 1.7 atmos. gage.

During the rectification of air, the rare gases and hydrocarbons become enriched by solution in liquid oxygen. The hydrocarbons may be removed either continuously or periodically when their concentration reaches a dangerous value by any one of a number of well-known methods, such as filtration by adsorption, catalytic combustion, or other chemical reaction carried out with the gaseous oxygen. The remaining mixture of oxygen, krypton and xenon is then separated in accordance with this invention by rectification under increased pressure and preferably under a pressure greater than about 1.7 atmos. gage. By continued rectification in this manner, krypton and xenon may be concentrated in solution in liquid oxygen to substantially any desired degree, and may thus be obtained in a relatively very pure state.

It will, of course, be appreciated that certain changes may be made in the hereindescribed method without departing fromv the principles or exceeding the scope of this invention as indicated in the appended claims.

I claim:-

1. Process for obtaining krypton and xenon by rectification which comprises separating the mixture of oxygen, krypton and xenon obtained at the first enrichment of the rare gases by rectification under increased pressure, said pressure being increased after the primary rectification and for said final separation only.

2. Process for obtaining krypton and xenon by rectification which comprises removing the hydrocarbons from the mixture of oxygen, krypton and xenon obtained at the first enrichment of the rare gases, and separating the mixture of oxygen, krypton and xenon by rectification under increased pressure, said pressure being increased after the primary rectification and for said final separation only.

3. Process for obtaining krypton and xenon by rectification which comprises separating the mixture of oxygen, krypton and xenon obtained 60 at the first enrichment of the rare gases by rectification under a pressure above 1.7 atmos. gage.

4. Process for separating rare gases from air which comprises subjecting the air to rectification, whereby a mixture of liquid oxygen and rare gases is obtained; and subjecting said mixture to rectification under increased pressure, said prev,

sin-e being increased alter the primary rectification and for said final separation only. I

,5. Process for separating krypton and xenon from air which comprises subjecting the air to rectification, whereby a mixture 01 liquid omen, hydrocarbons, krypton, and xenon is obtained; removing the hydrocarbons from said mixture; and subjecting the remaining mixture to rectification under increased pressure, said pressure being increased after the primary rectification and for said final separation only.

6. Process for separating krypton and xenon from air which comprises subjecting the air to rectification, whereby a mixture of liquid oxygen, hydrocarbons, krypton, and xenon is obtained; removing the hydrocarbons from said mixture; and subjecting the remaining mixture to rectification under a pressure greater than 1.7 atmpa.

gage.

7. Process for obtaining krypton and xenon by rectification which comprises separating the mixture of oxygen, krypton, and xenon obtained at the first enrichment of the rare gases by subsequent rectification under an increased pressure sumcientto keep the major elements of the mixture in the liquid phase.

8. The process for producing substantially to keep the oxygen, krypton. and xenon m in the liquid phase. 

